1. Field of the Invention
The present invention relates to the field of electrophotographics and more particularly to a printing device for recording information in the form of toner images.
2. Description of the Prior Art
U.S. Pat. No. 3,816,840 describes a printing device for recording information in the form of toner images. In particular, FIGS. 9 and 10 of this patent show a magnetic developing system constructed from a number of permanent magnets having a nonmagnetizable sleeve disposed rotatbly therearound and provided with magnetizable conductive toner. A fixed pin electrode is disposed opposite the toner particles on the sleeve in the recording region. A dielectric layer can be transported over the pin electrode which is always in contact with the pin electrode. A dotted toner image is deposited on the dielectric layer by applying short high voltage pulses (typically 500 to 1000 volts) between the sleeve and the pin electrode. The image dots have dimensions which are substantially equivalent to the size of the area of the pin electrode. By using a large number of such pin electrodes and actuating them with image-forming signals, a dotted toner image can be obtained image-wise on the dielectric layer.
A disadvantage of this printing device is that the image developed in this way has low resolution because recording is possible only in a dot-wise fashion. Also, it is technically very difficult to achieve the required contact between a fixed row of electrodes and a moveable dielectric layer.
U.S. Pat. No. 3,946,402 discloses a device in which a rotating drum is provided with a dielectric layer. A conventional magnetic developing brush is used to apply a uniform toner layer to the dielectric. The uniform toner layer is fed into an image-forming zone formed by a magnetic roller provided with a nonmagnetic sleeve. A large number of magnetic electrically conductive electrodes in the form of bars are disposed on this sleeve. The electrodes extend in a line parallel to the axis of the drum. The electrodes are each connected to a voltage source. If the electrodes are not electrically energized, the magnetic force will pull off the toner from the dielectric layer. Energizing the electrodes, with pulses corresponding to a desired image, forms an electric field across the dielectric layer and fixes the toner dotwise to the dielectric layer. Since the electrodes are conductive, they must be insulated from one another.
A disadvantage of this printing device is that the conductive toner particles may form an electrical connection between individual electrodes so that short circuiting occurs thereby interfering with the formation of a sharp clear image. Also, the construction of a row of conductive magnetic electrodes in bar form is very complex and expensive. U.S. Pat. No. 3,879,737 has the same disadvantages because it uses the same arrangement of magnetically conductive electrodes.
The printing devices of U.S. Pat. Nos. 4,390,887 and 4,394,671 and British Pat. No. 2,076,746 all have a similar problem. The electrically conductive stylus is placed in contact with the conductive toner particles between the surface of the drum and the dielectric layer. Because of this placement an electrical connection may form across the individual electrodes of the stylus so that short circuiting occurs, thereby interfering with the formation of a sharp clear image.
U.S. Pat. No. 3,739,087 and British Pat. No. 1,483,935 although disclosing recording devices cannot be adapted for use with electrically conductive, magnetic toner particles. These devices also have the short circuiting problem mentioned above and are limited to the dot-wise formation of images.
It is, thus, an object of the invention to provide a printing device for recording information in the form of toner images which does not have the above-mentioned disadvantages.